Schizophrenia is a debilitating disorder affecting the psychic and motor functions of the brain. It is typically diagnosed in individuals in their early to mid-twenties and symptoms include hallucinations and delusions or at the other extreme, anhedonia or social withdrawal. Across the spectrum, the symptoms are indicative of cognitive impairment and functional disabilities. Notwithstanding improvements in antipsychotic treatments, current therapies, including typical (haloperidol) and atypical (clozapine or olanzapine) antipsychotics, have been less than acceptable and result in an extremely high rate of noncompliance or discontinuation of medication. Dissatisfaction with therapy is attributed to lack of efficacy or intolerable and unacceptable side affects. The side effects have been associated with significant metabolic, extrapyramidal, prolactic and cardiac adverse events. See Lieberman et al., N. Engl. J. Med. (2005) 353:1209-1223.
While multiple pathways are believed to be involved with the pathogenesis of schizophrenia leading to psychosis and cognition deficits, much attention has focused on the role of glutamate/NMDA dysfunction associated with cyclic guanosine monophosphate (cGMP) levels and the dopaminergic D2 receptor associated with cyclic adenosine monophosphate (cAMP). These ubiquitous second messengers are responsible for altering the function of many intracellular proteins. Cyclic AMP is thought to regulate the activity of cAMP-dependent protein kinase (PKA), which in turns phosphorylates and regulates many types of proteins including ion channels, enzymes and transcription factors. Similarly, cGMP is also responsible for downstream regulation of kinases and ion channels.
One pathway for affecting the levels of cyclic nucleotides, such as cAMP and cGMP, is to alter or regulate the enzymes that degrade these enzymes, known as 3′,5′-cyclic nucleotide specific phosphodiesterases (PDEs). The PDE superfamily includes twenty one genes that encode for eleven families of PDEs. These families are further subdivided based on catalytic domain homology and substrate specificity and include the 1) cAMP specific, PDE4A-D, 7A and 7B, and 8A and 8B, 2) cGMP specific, PDE 5A, 6A-C, and 9A, and 3) those that are dual substrate, PDE 1A-C, 2A, 3A and 3B, 10A, and 11A. The homology between the families, ranging from 20% to 45% suggests that it may be possible to develop selective inhibitors for each of these subtypes.
The identification of PDE10 was reported by three groups independently and was distinguished from other PDEs on the basis of its amino acid sequence, functional properties, and tissue distribution (Fujishige et al., J. Biol. Chem. (1999) 274:18438-18445; Loughney et al., Gene (1999) 234: 109-117; Soderling et al., PNAS, USA (1999) 96: 7071-7076). The PDE10 subtype at present consists of a sole member, PDE10A, having alternative splice variants at both the N-terminus (three variants) and C-terminus (two variants), but that does not affect the GAF domain in the N-terminus or the catalytic site in C-terminus. The N-terminus splice variants, PDE10A 1 and PDE10A2, differ in that the A2 variant has a PKA phosphorylation site that upon activation, i.e. PKA phosphorylation in response to elevated cAMP levels, results in intracellular changes to the localization of the enzyme. PDE10A is unique relative to other PDE families also having the conserved GAF domain in that its ligand is cAMP, while for the other GAF-domain PDEs the ligand is cGMP (Kehler et al., Expert Opin. Ther. Patents (2007) 17(2): 147-158). PDE10A has limited but high expression in the brain and testes. The high expression in the brain and, in particular, the neurons of the striatum, unique to PDE10, suggests that inhibitors thereto may be well suited from treating neurological and psychiatric disorders and conditions.
Inhibition of PDE10 is believed to be useful in the treatment of schizophrenia and a wide variety of conditions or disorders that would benefit from increasing levels of cAMP and/or cGMP within neurons, including a variety neurological, psychotic, anxiety and/or movement disorders. Accordingly, agents that inhibit PDE10 and especially PDE10A would be desirable as therapeutics for neurological and psychiatric disorders.